The effect of temperature on the activities of acyl-CoA:lysolecithin acyltransferase and acyl-CoA hydrolase has been studied in microsomal preparations. The enzymes had different thermal stabilities, the hydrolase being more stable. The temperature dependence of enzyme activities was studied either in 4 M NaCl-washed microsomes or NaCl-washed detergent-treated microsomes. Both preparations showed little increase in acylation rate when the temperature was increased, whereas hydrolase activity was increased markedly. This increase in hydrolytic activity was higher when lysolecithin was absent. Based on this behavior of the enzymes, the relative accuracies of the spectrophotometric and radioactive assay methods are discussed.